The immature holometabolous insects are called larvae. These larvae go through several instars, or moults, each time growing larger until the pupal instar. The pupal instar marks the massivephysiological change between larvae and adult. Most notably, only adult insects have wings(if they have wings): larvae never have real wings. Pupation can occur in a cocoon, or with the insect open to the elements. It depends on the bug.

There are, of course, several reasons that such a complete metamorphosis evolved. The most obvious reason, that most of the 'higher' insect orders are holometabolous is food. Complete metamorphosis means that larvae and adults share different environments and have different mouth structures. This means they are eating different food.